The present invention relates to an apparatus for the separation of gas and liquid from a flowing mixture of gas and liquid. In particular, the invention relates to an improved gas/liquid separator of the cyclonic variety.
In various industrial processes, such as in washing wood pulp, it is advantageous to draw air from over a reservoir of liquid by means of a vacuum provided by a blower intake. The air is usually drawn through the blower and provided under pressure to a portion of an apparatus, such as a pulp washer, to create a region of positive pressure. However, the air drawn by the vacuum created by the blower may have liquid or solid particles entrained therein, particularly where the liquid has foam or froth covering its surface. For proper operation of the blower and the apparatus, it is necessary to separate the entrained particles from the air before the air is taken in by the blower.
Various means of separating a gas from a gas/liquid flow mixture are known in the art. In particular, separation of liquid and solid particles from gas streams by cyclonic action is known. For example, a contact-and-separating element of a vortex tray of a liquid-gas mass-transfer apparatus using cyclonic separation is disclosed in U.S. Pat. No. 4,838,906 to Kiselev. Similarly, a dust collection system using cyclonic separation is disclosed in U.S. Pat. No. 2,393,112 to Lincoln.
In a typical cyclonic separator, a gas mixture having particles entrained therein is drawn vertically upward. A cyclonic rotation is imparted to the flowing mixture, typically by means of helical or spiral vanes. Centrifugal force causes heavier particles to be forced radially outwardly toward the outer periphery of the flow mixture where the particles drop back downward under the force of gravity. The gas mixture without the heavier particles continues upwardly.
Cyclonic gas/liquid separators may be used in a flat bed wood pulp washer generally similar in construction and mode of operation to a Fourdrinier paper machine incorporating an endless foraminous belt (xe2x80x9cwirexe2x80x9d), a headbox which delivers the pulp suspension in a pulping liquor to one end of a horizontally traveling upper run of the wire, successive washing zones along the length of the run, and means at the downstream end of the run for receiving and removing the resulting washed pulp. Pulp washers of this type, manufactured by the assignee of the present invention in accordance with Ericsson U.S. Pat. No. 4,154,644 of 1979, have been notably successful, and the present invention was developed to improve the operation and results obtained by such pulp washers.
The operation of a pulp washer of this type may be described as being according to the displacement washing principle. That is, once the pulp mat has been formed, it is not rediluted, but simply is subjected to repeated washings by application on top of the mat of washing liquid with the liquid applied in each washing zone having a lower concentration of liquor than the filtrate from the preceding zone. The liquid applied in each zone enters the mat substantially en masse and thereby displaces the liquid which was carried into the zone in the mat and causes it to drain therefrom through the wire.
The mechanical elements of a washer according to the Ericsson patent include a hood which encloses the entire apparatus downstream from the headbox, and a series of receptacles below the operating run of the wire in sealed relation with the hood. In operation, vacuum is applied to the receptacles, and/or gas pressure is developed within the hood, to augment the action of gravity in forcing the washing liquid through the pulp mat on the wire. Gases and vapors drawn through the wire into the upper spaces in the receptacles are recycled back to the hood to increase the pressure differential above and below the wire.
In the pulp washer, a gas/liquid separator or mist eliminator is attached to each of the receptacles at a gas outlet on top of the receptacle to thereby permit the gases and vapors to be drawn from the flat top receptacles through the mist eliminator to the suction side of a pump or fan that recycles the gas, in this case air, to the hood. However, entrained within the gases and vapors drawn from the receptacle are particles of mist and foam from the space between the wire and the pulping liquor.
Prior art mist eliminators employing cyclonic separating means have been used to remove particles of mist and foam from the gas flow before the gas reaches the pump or blower. The typical mist eliminator has a cylindrical housing arranged vertically proximate the top of the receptacle with a vacuum line attached to the top end. The gas inlet is positioned above the level of the liquor with cyclonic-flow-inducing means, typically helical vanes or a swirler, positioned within the inlet. Droplets of mist and particles of foam are hurled radially outwardly by centrifugal force from the gas/liquid flow to drop back down into the liquor under the force of gravity.
However, in many cases the upward draft of gas through the mist eliminator impinges upon the falling, separated liquid droplets. This tends to retard the separation of undesired droplets of mist and particles of foam entrained within the gas flow. Thus, incomplete separation of foreign matter from the gas results. Mist and foam entrained within the gas flow result in a lower pressure differential being developed between the hood and the receptacle, thereby reducing the efficiency of the pulp washer.
In accordance with the disclosure of WO Publication 98/29179 (of common assignment herewith) an improved mist eliminator is provided wherein an annular zone is formed in the separator where falling droplets of separated water and liquid drain downwardly in the device substantially without resistance from countercurrent flow of gas and liquid.
Although the mist eliminator set forth in the aforementioned WO Publication has proven commercially successful, it was found that in some instances, where dense foams were encountered in the suction box, turbulence was actually increased in the liquid directly beneath the separator, leading to inefficient separation. Accordingly, there is a need in the art for a mist eliminator device of enhanced efficiency that is capable of separating foam components that may exist in the suction box.
These and other objects of the invention are met by the provision of a cyclonic separation device wherein two distinct cyclonic action zones are provided to perform the desired separation of the liquid components from the gas component. The device includes a generally elongated cylindrical housing with inlet end and outlet end disposed at opposite axial ends of the cylinder. Within the cylinder and at an approximate medial position along the longitudinal axis of the housing, a swirl imparting vane structure is located. This vane structure forms a boundary defining a upstream cyclonic zone and downstream cyclonic action zone. The location of the swirl imparting vane structure at an approximate mid-point along the length of the cylinder contrasts with many prior art designs where the swirl vanes are located adjacent the inlet end of the separator.
The outlet of the housing is connected to a suitable suction source which draws the gas/liquid mixture to be separated into the inlet portion of the housing, passing generally upwardly through the housing in an upstream to downstream direction. The mixture is first separated in the upstream cyclonic zone with an additional separation performed in the downstream cyclonic action zone.